


Something About Prime Numbers

by neyla9



Series: Trustworthy AU [4]
Category: Gravity Falls
Genre: M/M
Language: English
Status: Completed
Published: 2016-03-08
Updated: 2016-03-08
Packaged: 2018-05-25 14:30:03
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,116
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6198691
Author URL: https://archiveofourown.org/users/neyla9/pseuds/neyla9
Summary: <blockquote class="userstuff">
              <p>Bill has been acting strange and suddenly says he need to go to the mindscape for a month. Dipper is worried about his lover and wants to help him anyway he can.</p>
            </blockquote>





	Something About Prime Numbers

**Author's Note:**

> Okay, so this was somewhat requested by Rockin_Robyn02. They originally requested for a Mewberty-like fic, but I decided not to use puberty, since Bill has already fully matured in my Trustworthy Au. Anyway, I originally said I was going to wait to post this after a certain point in the main Trustworthy Au story, in order to avoid spoilers, but I couldn't wait, so I wrote out all the spoilers insteadX3

It happened towards the end of March when the twins were sixteen; everything had started out as an average morning, Dipper had cooked breakfast, Mabel was working on a new knitting project and had insisted on continuing her work at the table. Bill, however, had slept in late, which was unusual for the dream demon. Dipper had tried to wake the demon up, only getting a couple of mumbles in response.

Dipper was making scrambled eggs while waiting for the bread to finish toasting when Bill came into the kitchen.

“Morning,” he greeted the twins with a yawn and hugged Dipper from behind.

“Morning, Bill,” Dipper replied while the demon began snuggling into the human’s neck.

“Well, somebody’s affectionate today,” Mabel laughed. Bill didn’t seem to react, but he did grab both of Dipper’s arms.

“Bill,” Dipper sighed in frustration. “I’m going to need free range of my arms to make breakfast,” Dipper paused for a moment when he noticed something. “Bill, your hands are really dry…”

“Hmm?” Bill perked up at that.

“Oh, Wendy gets dry hands too,” Mabel told the demon. “She’d probably give you some of her moisturizer if you asked her.” Bill’s grip loosened somewhat, allowing Dipper to take the opportunity to free his right arm, after which he stroked the demon’s hand.

“They’re really dry,” Dipper remarked. “They’re practically flaking,” as Dipper noted that, a bit of dry skin peeled off of Bill’s hand, revealing something black underneath. “What’s that?” Dipper tried to touch the black substance, but Bill quickly pulled away from the human, even taking a few backwards steps.

“Nothing,” Bill insisted. “Um, what year is it?”

“… 2017,” Dipper responded slowly.

“Damnit,” Bill cursed under his breath, before beginning to back out of the kitchen, a fake smile plastered on his face. “Listen, Pine Tree, I’m gonna be away all of April, okay?”

“Why?” Dipper asked.

“Nothing,” Bill repeated. “Just… personal things… Nothing you need to worry about. I’ll just be in the Mindscape for the next month, okay? Great! I’ll go up and pack some stuff and leave tonight!”

And with that, Bill teleported away.

 

Dipper was puzzled by the demon’s behavior, so he decided to ask the only person who could possibly know what was wrong with the demon.

“Grunkle Ford?” Dipper called as he entered the lab; Ford seemed busy working on a new experiment.

“Hi Dipper,” Ford greeted him with a small smile. “What are you doing down here?”

“Well, Bill’s acting stranger than usual, and I’m kinda worried about him,” Dipper explained.

“Did he try to burn down another church?” Ford asked deadpanned.

“No, nothing like that,” Dipper quickly assured his uncle; none of them wanted a repeat of what had happened when a pastor had called Bill and Dipper “Godless heathens” because the two had held hands in public. “He was really tired this morning, and his skin was incredibly dry and it peeled away and underneath there was this black material… You’ve known Bill for a long time; has something like that ever happened before?”

“None of the symptoms you listed sound familiar,” Ford responded with a thoughtful expression. “Maybe it’s a side effect from his physical body?”

“I don’t know,” Dipper sighed. “Then, has he ever had to leave for a month at a time?”

“No,” Ford shook his head. “Sometimes he would be gone for a couple of days, but never more than a week… That is, until the activation of the portal, of course…”

“I’m worried sick about him,” Dipper confessed. “What if it’s some kind of demon disease and he’s hiding away because he doesn’t want me to catch it? What if it’s fatal? Or what if-“

“Maybe you should talk to Bill about it?” Ford suggested.

“You’re right,” Dipper nodded and made to leave the lab. “Thanks, Grunkle Ford. I’ll let you know if I find anything.”

 

Dipper went up to his and Bill’s shared room, but found it empty. Alright, so the demon wasn’t there now, but he was bound to turn up sooner or later. Dipper sat down on the bed, but after fifteen minutes had passed, he grew somewhat bored, so he pulled out the journal and started reading.

However, once two hours had passed and there was still no sign of the demon, Dipper closed the journal and set it aside. He sat down in the chair by the desk and placed his head on said desk. He blinked a couple of times until his eyes focused on a yellow piece of paper that Dipper hadn’t spotted when he came in. Dipper didn’t know if he had simply missed it or if Bill had teleported it there. Either way, he grabbed it and read it:

‘Dear Pine Tree,  
I’m sorry I had to leave so sudden.  
I promise I’ll be back once April is over.  
It has nothing to do with you; this is just something I have to get past myself.  
Please, don’t try to find me inside the Mindscape.  
Love, Bill.’

So Bill was already in the Mindscape? Dipper was furious; why couldn’t Bill just explain what was going on? Why couldn’t the demon say goodbye face to face?

Dipper vaguely noticed that a small part of the letter was wet, stained with a thick, black liquid. Now Dipper was filled with concern as well.

He decided to disregard the letter’s warning and go into the Mindscape to find Bill. Luckily, the demon had taught him how to go there.

Dipper lied down on the bed, closed his eyes, and concentrated his magic, letting it pour over him and reach everywhere, each finger, each toe, even each individual strain of hair.

When he reopened his eyes, he found himself inside the Mindscape. He began walking around, using the traces of Bill’s magic as a trail, until he finally tracked down the demon who was standing in front of a massive double door.

“Bill!” Dipper shouted and ran towards the demon. He noticed that Bill was in his more human form rather than his demon form, which meant that Bill had phased his entire being into the mindscape rather than just his mind.

Bill seemed surprised and even a bit frightened at seeing Dipper running towards him, but the demon made no move to run away.

“Pine Tree,” Bill gasped as if he was out of breath, his pupils were more dilated than normal and for some reason he had his entire left arm completely covered by a black glove and his sleeve. “Didn’t you get my note?” the demon attempted to smile, but it was clearly strained. “You should leave… right now!”

“I’m not going anywhere,” Dipper crossed his arms in defiance. “I want to know what’s going on! I’m worried about you!”

“There’s no need for you to worry, Pine Tree,” Bill insisted, his smile becoming even more strained.

“Yes, there is! You’re acting really odd; you keep dodging my questions, you left without even saying goodbye, just a note saying that you won’t be back until May… And what’s wrong with your arm?!”

“Nothing!” Bill tried to pull his arm behind his back, but Dipper managed to grab it before the demon did so. As he grabbed Bill’s arm, the arm began to bend under Dipper’s grip, shaping to fit the human’s grip like a piece of clay. Dipper ripped the glove off, revealing that the entire arm was now covered in the black substance from before, or maybe had transformed into the black material.

“Bill, what is this?” Dipper asked, shaking slightly, his eyes wide at the sight.

“Stop it, Pine Tree!” Bill growled and violently pulled his arm away from Dipper. “There’s nothing you can do about it! You need to leave NOW! Before you get hurt!”

“Bill, I want to help you,” Dipper told his lover.

“You can’t help me!” Bill opened the door closest to him and slipped through it, slamming it shut before Dipper had a chance to follow. “Just leave, Pine Tree, PLEASE! You can’t stop this…”

Dipper almost tried to open the door, but stopped.

“Alright,” Dipper placed a hand on the door. “I’m going to trust you, Bill… Please, be safe…”

And with that, Dipper left the Mindscape.

 

A week later, April had begun and Dipper hadn’t heard a single word from Bill, not even another note. He was taking Bill’s absence rather hard, and the others noticed it. The next week went by with each of them trying to make him feel better.

Mabel made her brother a cozy, yellow sweater and suggested new things they could study for the journal. Grunkle Stan gave him extra work around the shack, probably to distract Dipper without giving off the impression that the old man cared. Wendy rented a bunch of horror movies and had a movie marathon with Dipper. Soos tried to give his nuggets of wisdom, as well as promising that he was there for Dipper if he wanted to talk about it. Grunkle Ford tried to help by asking for Dipper’s help with a scientific project.

Dipper appreciated his friends and family’s attempts to make him feel better, but once the day was over and he was lying in that big, cold bed, all alone, none of them could help him.

The worst parts were the dreams; usually Dipper would either have nightmares or Bill would ward off the nightmares with pleasant dreams, but ever since Bill left, all Dipper had were dreamless sleeps that left him tired and depressed.

In the end, Dipper couldn’t take it anymore and made a decision.

“I’m going into the Mindscape,” he told Mabel. The two of them were alone in the living room; Mabel had been watching a documentary about glitter when Dipper walked in to tell her the news.

“Dipper,” his sister gave him a worried yet sympathetic look. “Are you sure this is a good idea?”

“Probably not,” Dipper sighed. “But I can’t go on like this until May; I keep worrying about Bill, and I miss him like crazy… I just wanted to tell you before I left.”

“Don’t you think you should tell Grunkle Stan or Grunkle Ford?” Mabel asked as her brother as he began walking up the stairs.

“They would just try to stop me,” Dipper responded, pausing on the steps. “I’ll be fine, Mabel,” he gave his sister a reassuring smile. “I’m not completely helpless, you know.”

And with that, Dipper walked up the last steps and into his and Bill’s room.

 

When Dipper arrived in the Mindscape, it didn’t take him long to relocate the door Bill had gone through; it just stood there, in an empty void, seemingly leading to nowhere. The door was rather big, made from dark wood, with golden handles. Dipper noticed that many something had been carved into the door, carvings that he swore hadn’t been there when Bill entered, the main focus in the pattern being a triangle with a single eye; Bill’s symbol.

Dipper tried to open the door and found that it wasn’t locked. He peeked inside and saw a long, wide, dark hallway, the air inside seeming somewhat humid. Dipper stepped inside, his shoes clicking on the hallway’s stone floor.

As he walked further down the hallway, he heard the creaking of the door closing behind him, but he didn’t turn around to look. Each step he made echoed through the long corridor as he kept walking forward, until he stepped in something. Dipper looked down and saw that he had stepped in some black slime, similar to what had covered Bill’s arm. Dipper realized that the floor was covered in a trail of the dark substance, leading down the hallway.

Dipper walked towards the left wall to avoid stepping in the slime, but when his hand touched the wall, he realized that it too was covered in the stuff; in fact, it was sliding down from the wall to the floor. Dipper pulled his hand away, dried off the slime on his pants and continued moving forward. He heard a shuffling sound from above, before he felt something slide down his neck and under his shirt. He used his hand to remove whatever had gotten down there, and it turned out to be more of black slime that had dripped down from the ceiling. Dipper looked up before continuing to walk on, but he couldn’t see anything aside from a black abyss.

“Bill?” Dipper called out as he walked back down the hall, his echo sounded down the corridor. “Are you still in here?”

After a bit more walking, Dipper encountered a dead end; the hallway ended in a giant, white, marble wall. Dipper did find it odd, however, that everything else was covered in the black slime, yet this one wall was completely clean. He walked towards the wall, stretching out his hand to touch it, but right as his hand made contact, something swooped down from the ceiling and tackled him to the floor, holding him down.

The creature was big, but its body structure was slender. It had three long, thin arms, with fingers that looked more like long thin claws than fingers, a wide mouth filled with big, sharp teeth and a long, thick tongue, as well as a second mouth on its stomach even bigger than the one on its face, its long, messy hair was a dull yellow that reached its mid back, the structure of the hair more similar to that of a hyena’s mane than any human hair. It didn’t have a nose, just one big, glowing, yellow eye with a slit pupil. The creature’s entire body seemed to either be covered in, or made of, the thick, black slime.

“Bill?” Dipper gasped staring into the creature’s eye. There was no doubt about it; Dipper would recognize Bill anywhere, especially his eyes, or in this case, eye. “What happened to you?”

Bill didn’t respond; he was glaring at Dipper, growling. As Bill moved his face closer to Dipper, the human turned his face away to try and distance himself from the demon, exposing his neck in the process. The demon seemed to pause at that, stopping his growling. Instead, he moved closer to the human’s neck, beginning to sniff it, despite the lack of a nose.

“Bill? What’s going on?” Dipper asked, knowing he wouldn’t get an answer; whatever had happened to Bill seemed to have removed the demon’s ability to speak. Dipper froze as he heard Bill beginning to make a purr-like noise.

The demon rose from the floor, freeing the human from his grip. Bill walked over to the blank wall and began to crawl on it. He looked over his shoulder, apparently to see if Dipper was watching him, which the human was, before the demon began to pull threads of the black substance from his palms and started to weave it around in patterns on the wall.

He continued like that for quite a while, Dipper watching the entire process, before the demon apparently finished, the end result reminding Dipper somewhat of a spider web.

The demon climbed back down to the floor and picked up Dipper using his many arms. Bill then proceeded to try and stick Dipper on the wall, but after Dipper fell off, the demon seemed to realize that humans couldn’t stick to the wall all by themselves. So Bill used one pair of arms to keep Dipper in place on the wall, the other pair to pull out the black slime-thread, and the last to tie Dipper up, hands and legs, to the black web.

Dipper was beginning to fear that the demon was going to try and eat him.

Bill began licking Dipper’s neck, his long tongue tickling the human. The demon then proceeded to give Dipper’s neck a small bite, which caused Dipper to yelp out in pain.

Bill paused at that, but soon continued biting the human’s neck, only this time around, his bites were more like careful nibbles.

“B-Bill!” Dipper gasped as the demon’s hands began roaming his body. The hands then began to rip off every article of clothing that Dipper was wearing, including his hat and shoes, leaving the human naked. Bill pressed his body closer to Dipper, as if the demon was trying to absorb the human’s body into his own. Around Bill’s nether regions, a tentacle-like appendage began emerging, and slithered up Dipper’s leg.

Bill stuck his tongue into Dipper’s mouth in a way that somewhat resembled a kiss, mapping out the insides of the human’s mouth.

Meanwhile, the slick appendage attached to Bill’s crotch swiftly entered Dipper, causing the human to cry out in pleasure, but each moan and gasp was swallowed by Bill. Dipper could feel the appendage grow bigger and thicker inside him, starting to pump in and out of him, hitting all the right spots. It didn’t take long before Dipper was brought to his climax, but the demon kept going, the tongue on Bill’s stomach licking up the mess from Dipper’s orgasm. Soon, Dipper was hard again, despite his exhausted body.

“B-Bill…” Dipper moaned as the demon finally broke away from the kiss, only to started nibbling Dipper’s neck again. The demon made a small growling noise in response as his thrusts became harder, until Dipper felt a wave of something filling him, the sudden rush bringing Dipper to his second orgasm.

It continued like that for Dipper couldn’t tell how long; Bill had told him before that time worked differently in the Mindscape, the same way time works differently in a dream. It could either have been a couple of minutes or it could’ve been several days, all Dipper knew was that the entire time had been spend with the two of them having sex until they were too exhausted to go on, then resting, and then starting again.

By the time Dipper woke up after a long string of orgasms, expecting Bill to pounce him again, he found instead that he was lying on the floor, none of the black slime in sight, and a naked, normal-looking Bill was lying on top of him, sleeping.

“Bill?” Dipper lightly shook the demon’s shoulder.

Bill responded with a small, tired sound, before slowly opening his eyes. At first the demon seemed slightly confused when he saw Dipper, but then his confusion morphed into a face of terror, as he grabbed Dipper’s arms.

“When did you get here!?” Bill shrieked at the human.

“I-“ Dipper was interrupted by Bill spotting the bruises on Dipper’s neck.

“Did I do this to you?” Bill stroked the bruises gently. “Oh, Pine Tree, I told you not to come here!”

“Bill, are you gonna tell me what happened or not?” Dipper snarled at the demon.

“First, you need to tell me what I did to you,” Bill insisted. “Did I hurt you in any other way?”

“Bill, you didn’t really hurt me,” Dipper told the demon. “I think… you fucked me? But I’m not too sure… You looked and acted really different.”

“I… Are you sure I didn’t hurt you?” Bill looked intently Dipper.

“Yes,” Dipper promised. “If you had hurt me, you wouldn’t hear the end of it, trust me. Now, please explain to me what happened.”

“Alright,” Bill took a deep breath before starting his explanation. “You know how I told you that demons aren’t very social creatures? Well, because of that asocial societal structure, once every couple of years, for one month, all fully matured demons go into… heat. When a demon enters heat, there are three different behavioral patterns they can choose to undertake; Provider, Carrier, and Challenger. A Provider will find somewhere, like this room, and along with one or more Carriers, mark it as their domain. A Challenger can attempt to take away a Provider’s domain and Carriers, but the Provider will fight furiously over it, and often times, Carriers will fight off the Challenger too, unless they prefer the Challenger over the Provider. If the Challenger loses, they’re killed, but if they win, they become the new Provider, and the old Provider is either killed or chased off. The Carriers are the ones who get pregnant from the heat, and the Provider must help them through the pregnancy. And, that’s pretty much it.”

“So, what? Are you telling me, you were in heat?” Dipper asked.

“Yep,” Bill responded awkwardly, refusing to look at Dipper.

“So… Had you found another demon to spend your heat with?” Dipper questioned somewhat carefully.

“No, of course not!” Bill scoffed at the idea.

“Then, you were just going to wait it out?”

“Yeah,” Bill shrugged. “Which is why I told you not to come.”

“If you had just told me about it, I could’ve helped you from the start,” Dipper argued.

“Pine Tree, when I’m in heat, I have no control! You’re a human! I could’ve accidently killed you! In fact, it’s a miracle you’re still alive, considering you entered my domain after my heat had started; I would’ve taken that as a challenge.”

“I think you did, actually; I touched that big wall, and you jumped me.”

“You touched my mating slate and I didn’t kill you!?!” Bill stared at Dipper with wide, fear-filled eyes.

“’Mating slate’?” Dipper gave the demon a look.

“That’s what it’s called, Pine Tree, I didn’t come up with the name.”

“Well, anyway, you seemed really angry at first, but then you started sniffing me and purring…”

“Huh,” Bill thought for a moment. “I… I don’t know how that’s even possible.”

“Well, I think you recognized me, even during your heat,” Dipper gave the demon a small smile.

“What did I do next?”

“You… started weaving like, a web on the… mating slate, and then you tried to stick me on it, but I wouldn’t stick, so you tied me to it…”

“Okay…” Bill gave a small nod. “Let me guess, and then we started to have sex?”

“Whatever gave you that idea?” Dipper said sarcastically. “But, yeah, we did… many, many times…”

“Pine Tree, what I’m about to say may surprise you, but… you and I… mated.”

“I… had kinda guessed that halfway your explanation,” Dipper responded. “You know, I could’ve avoided a lot of worrying if you had just told me from the beginning.”

“Maybe, but then you’d have insisted on helping me-“

“Exactly!” Dipper interrupted. “Look, Bill, we’ve been together for a while, and when you’re in a relationship, you give your partner what they need; you needed this, so I helped you. And it’s not like I was totally against this… I might be up for it again the next time you go through a heat.”

“Yeah, you’re saying that now,” Bill gave Dipper a small kiss on the cheek.

“Come on, Bill, it’s not like anything bad happened.”

“Well, aside from the fact that you’re pregnant now…” Bill trailed off.

“WHAT!?!” Dipper shrieked, his entire body jolting in surprise.

“Just kidding,” Bill smirked. “You’re a human with a masculine sex organ! You know those can’t get pregnant!”

“You’re such a jerk,” Dipper glared at the demon and gave him a small punch.

“I love you too, Pine Tree,” Bill laughed and held Dipper close.

Dipper scowled for a bit more, before he calmed down and nuzzled close to the demon.

“This is kinda nice,” Bill hummed. “I think this is the first time my heat has ended with cuddling.”

“Oh, right, you’ve been in heat before…”

“It’s a biological process, Pine Tree, I can’t just choose not to.”

“I get that, it’s just… I mean, do you have children?”

“What? No! Look, I wasn’t exactly the most popular demon back home; my options were limited. Most heats, I couldn’t even find a single partner, and the few times I did, my partner chose to eliminate the pregnancy…”

“I’m sorry to hear that, Bill,” Dipper gave the demon a sympathetic look.

“Don’t worry about it, Pine Tree; I don’t consider it a huge loss.”

“Well, on the bright side, it seems we have more in common than I thought,” Dipper smiled at the demon. “So, when’s your next heat anyway?”

“In ten years,” Bill responded.

“Then it’s once a decade?”

“No, it’s a little more complicated than that; it’s prime numbers,”

“’Prime numbers’?” Dipper gave the demon a confused look.

“Every time the year is a prime number, it means that a heat will happen. 2017 is a prime number, and so is 2027.”

“Hmm,” Dipper pondered for moment before placing a hand on Bill’s stomach. “You know, Bill, a lot of things can happen in ten years…”

“Yeah,” Bill replied with a small smile. “It can…”

**Author's Note:**

> Well, hope you liked it:3 But while I'm here, I also decided to post this earlier for another reason; I know a lot of people reading my Trustworthy Au is looking forward to the Dungeons, Dungeons, and More Dungeons chapter, and I'm sorry, but I couldn't write it! And that puts me in the decision on either putting Trustworthy Au on hiatus until I get it done or to not write Dungeons, Dungeons, and More Dungeons and move on to the next chapter.
> 
> Since I won't make my readers wait so long for a chapter, which might not even turn out that good and doesn't even contain any important plot elements to the story, I've decided to move on to the next chapter instead. I'm sorry.


End file.
